64 Regulation of Size in Unicellular Organisms 



lates of a single clone as food) the variability was that 

 shown in figure 20 above. Each distribution shown 

 is based on measurements made on two to four suc- 

 cessive days, and there was little difference in the dis- 

 tributions on each of the several days. The progeny of 

 a single individual did not vary so widely as the pro- 

 geny of all cousins or of all second cousins or of all 

 daughters. But there were also slight differences be- 

 tween the sets of animals from different culture dishes ; 

 showing that with the utmost control of environmen- 

 tal conditions, environmental effects were not wholly 

 excluded, though it happened that the environmental 

 effects averaged out upon successive days. It is im- 

 portant to note that the range of variation was just as 

 great among the animals in a single dish in the second 

 generation from a single ancestor as in a large clonal 

 population. There seems no doubt, therefore, that 

 most of the variation is unavoidable by any possible 

 isolation of the immediate ancestors. 



The effect of conjugation upon the inheritance of 

 size was not studied in Colpoda. Encystment in pro- 

 tective sheaths involves a considerable reduction of 

 body volume which is largely accounted for by the ex- 

 penditures in digestion and evacuation. After emerg- 

 ing from encystment the same sizes were shown by the 

 animals as before it, when cultivated in the same way. 



3. Shelled Rhizopods 



In contrast to the organisms so far considered, shel- 

 led organisms have a permanent size between one re- 

 production and the next. Although the size of the 

 shell is not in reality the size of the organism, it is un- 

 doubtedly a proper measure of the organism at the 

 time when the shell formed. At reproduction there 

 is a rapid and great swelling of the protoplasm, so 



